The present invention generally relates to the field of oil and gas production and, more particularly, to methods and systems for determining the orientation of natural fractures excited or reopened during hydraulic fracturing treatments.
Seismic data is used in many scientific fields to monitor underground events in subterranean rock formations. In order to investigate these underground events, micro-earthquakes, also known as microseisms, are detected and monitored. Like earthquakes, microseisms emit elastic waves—compressive (“P-waves”) and shear (“S-waves”), but their spectral content peaks at much higher frequencies than those of earthquakes and generally fall within the acoustic frequency range of 100 Hz to more than 2000 Hz.
Standard microseismic analysis techniques locate the sources of the microseismic activity during hydraulic fracturing. In many gas fields, permeability is too low to effectively produce gas in economic quantities. Hydraulic fracturing addresses this problem by intentionally creating fractures in the gas fields that provide conduits to enhance gas flow. Fluid is pumped into wells at sufficient pressure to fracture the rock. The fluid also transports a propping agent (also known as “proppant”) into the fracture. The proppant, usually sand or ceramic pellets, settles in the fractures and helps keep the fracture open when the fracturing operation ceases. Production of gas is accelerated as a result of improved capability for flow within the reservoir.
Microseismic detection is often utilized in conjunction with hydraulic fracturing techniques to map created fractures. A hydraulic fracture induces an increase in the formation stress proportional to the net fracturing pressure as well as an increase in pore pressure due to fracturing fluid leak off. Large tensile stresses are formed ahead of the crack tip, which creates large amounts of shear stress. Both mechanisms, pore pressure increase and formation stress increase, affect the stability of planes of weakness (such as natural fractures and bedding planes) surrounding the hydraulic fracture and, therefore, cause them to undergo shear slippage. It is these shear slippages that generate weak seismicity.
The sources of the microseisms are detected with multiple receivers (transducers) deployed on a wire line array in one or more offset well bores, which are displaced from the treatment well in which the fluid is pumped. These offset well bores are also known as monitor wells. With the receivers deployed in several wells, the microseism locations can be triangulated as is done in earthquake detection. Triangulation is accomplished by determining the arrival times of the various p- and s-waves, and using formation velocities to find the best-fit location of the microseisms. However, multiple offset wells are not usually available. With only a single nearby offset monitor well, a multi-level vertical array of receivers is used to locate the microseisms. Data is then transferred to the surface for subsequent processing to yield a map of the natural fracture geometry and azimuth.
The local recovery rate from a treated well is influenced by, among other things, the orientation of the natural fractures within or in close proximity to the zone of elevated pore pressures created during the stimulation by hydraulic fracturing. Thus, reliable information concerning the orientation of these natural fractures can be important in assessing the results of the treatment, as well as in assessing the well's future performance.